law, as Descartes expressed it, states that the sine of the angle
of incidence bears a fixed ratio to the sine of the angle of
refraction for any given medium. Here, then, was another
illustration of the fact that almost infinitely varied phenomena
may be brought within the scope of a simple law. Once the law had
been expressed, it could be tested and verified with the greatest
ease; and, as usual, the discovery being made, it seems
surprising that earlier investigators--in particular so sagacious
a guesser as Kepler--should have missed it.

Galileo himself must have been to some extent a student of light,
since, as we have seen, he made such notable contributions to
practical optics through perfecting the telescope; but he seems
not to have added anything to the theory of light. The subject of
heat, however, attracted his attention in a somewhat different
way, and he was led to the invention of the first contrivance for
measuring temperatures. His thermometer was based on the
afterwards familiar principle of the expansion of a liquid under
the influence of heat; but as a practical means of measuring
temperature it was a very crude affair, because the tube that
contained the measuring liquid was exposed to the air, hence
barometric changes of pressure vitiated the experiment. It
remained for Galileo's Italian successors of the Accademia del
Cimento of Florence to improve upon the apparatus, after the
experiments of Torricelli--to which we shall refer in a
moment--had thrown new light on the question of atmospheric
pressure. Still later the celebrated Huygens hit upon the idea of
using the melting and the boiling point of water as fixed points
in a scale of measurements, which first gave definiteness to
thermometric tests.